[ATM] diagonal flat

[Otterstedt at t-online.de (Heiner Otterstedt)]

Hi Jerry,

>I started polishing a larger diagonal for my 16".  The first one I made was
>too small.  As I polished I noticed that the glass was clearing from the
>edge inward, with a 1 1/2" area in the center that did not polish out well.
>I am assuming that this is a cavity and I wasted my time. Am I correct?
>  
>
Either your pitch lap or your glas (or both?) are not realy flat, hard 
to say which one.
Did you grind your glass against two other flats before you started 
polishing?
How did you press your pitch  lap?

If you've got a good reference flat you could do Newton's Interference 
Test, as
Bob recommends.

Ciao, Heiner